Prior art machines for making filter bags from a web of filter paper comprise a plurality of stations, including a station for feeding a web of filter paper, a folding station and a first station for longitudinally sealing the folded web to form a continuous tubular wrapping of filter paper and preferably for transversally sealing the continuous tubular wrapping to form a succession of filter bags of substantially flattened shape.
The longitudinal and transversal sealing station comprises a specially shaped upper roller with tracks for longitudinally and transversally sealing the continuous tubular wrapping of filter paper. The seals may be made by heat sealing means or, in the solutions of the latest generation, by sonotrodes, positioned in contact with the product to be sealed.
In practice, the sonotrode is the transducer element that converts the power supply into vibrations that are discharged onto the product to be sealed.
Current sealing units based on sonotrodes usually comprise a pair of sonotrodes placed side by side to enable both seals to be made. The sonotrodes are positioned so that they face the roller and are mounted on a rigid structure so as to keep the gap between each sonotrode and the filter paper as constant as possible.
Indeed, the Applicant has found that the relative position between the emitting surface of the sonotrode and the product supporting surface constitutes a fundamental variable for good sealing quality.
Current sealing units based on sonotrodes, however, involve highly rigid systems of supports with a multiplicity of sensors and devices for controlling the force applied to the sonotrodes which make it extremely difficult to keep this relative position firm and constant.
Indeed, the system is unsuitable for very thin materials such as filter paper since the rigidity of the supports and the high number of external control variables for maintaining the distance between the sontrode and the product create vibrations resulting in distances that are larger than acceptable for a good quality seal and slowing down sealing operations themselves on account of control and feedback between the sonotrodes and the control system.
The present invention therefore has for an object to overcome the above mentioned drawback through a unit that enables the sealer sonotrode to apply a substantially constant contact force on the product to be sealed, while at the same time allowing high operating speeds and reducing costs.